Think about the last time you exercised. Was it at a high intensity? Or was it at a leisurely pace? Chances are if it was at high intensity you were left gasping for air, or more specifically oxygen afterwards. You might gasp for a minute or perhaps 3 minutes to get oxygen into your body for your muscles. However, even after this time has elapsed and you feel your breathing is normal, there’s a good chance that you are still taking in higher volumes of oxygen than normal. This is excess post-exercise oxygen consumption, often abbreviated as EPOC.

Excess post-exercise oxygen consumption can be defined as the volume of oxygen consumed during recovery, above that which would have been consumed at rest during the same time.

What Is Excess Post-Exercise Oxygen Consumption?

When working at low intensities, your body, or more specifically your lungs are able to provide enough oxygen for your muscles to respire and release energy (to work). This is known as aerobic respiration. An example of this would be walking, doing household chores or a leisurely jog or bike ride. Enough oxygen gets to your muscles to release the energy to carry out the work required of them.

But let’s up the intensity now.

When working at high intensities, you’re breathing heavily, filling your lungs which as much air and therefore as much oxygen as possible. An example of this would be a 400m sprint. But what if this oxygen is not enough to meet the demands of the muscles? You continue working anyway, finishing off that 400m sprint but by then your muscles are screaming for oxygen. You stop and gasp and gasp and gasp.

Here’s what happens. To start with your body had enough oxygen for the muscles to release energy for the muscular contractions used in the 400m sprint. Your body was working aerobically. However, at some point in the race, to keep your muscles contracting and pushing you forwards with such force, your body couldn’t get enough oxygen to the muscles to release energy via aerobic respiration.

The result?

Insufficient oxygen is being delivered to the muscles, so energy is released anaerobically. As a result, an oxygen debt is created, also known as your submaximal oxygen deficit.

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Graph showing how oxygen debt is repaid during excess post-exercise oxygen consumption (EPOC)

As soon as you stop, oxygen flushes into your lungs, quickly diffuses into your blood, and is sent to your muscles. During the time you were sprinting you were in an oxygen deficit. Now your body wants to replenish that deficit by breathing more oxygen in than normal during rest. This is why you tend to gasp heavily after a bout of anaerobic exercise.

Excess Post-Exercise Oxygen Consumption: The Afterburn Effect

One fitness trend which has stayed for quite a while is HIIT (high-intensity interval training). It involves short intervals of intense work periods followed by short rest periods, typically in a 1:1 ratio although it varies depending on the training. HIIT is often touted as a great way to burn fat, tone up and reap the benefits of cardiovascular exercise. I believe all to be true.

HIIT is great for toning up because many muscle groups are engaged during exercise, and since the recovery is short HIIT actually works both your aerobic and anaerobic systems. Double whammy!

Then there’s the idea that HIIT burns fat. Here’s how it works. During HIIT, the harder you work, the larger the oxygen deficit created. This is because you are working more anaerobically at higher intensities, and so you are working without oxygen.

After exercise, EPOC occurs in order to repay that oxygen debt. As a result breathing rate and heart rate remain elevated for a period of time after intense anaerobic training to help supply your muscles with oxygen. The effectively means you are respiring more during rest, so more energy is released. Fat is the preferred source of fuel at lower intensities, and since you have finished exercising, your body oxidises (breaks down) fats to provide the glucose for the mitochondria (which release energy) in the muscles to use in respiration.

This means you are actually burning more calories than normal after exercise, even when sitting around. This effect typically lasts between 2 – 10 hours depending on the intensity of the workout, but some sources cite EPOC can last up to 48 hours. I believe 24 hours to be a more reasonable suggestion if you went for a full-blown anaerobic workout with little recovery.

Most people tend to overestimate the number of calories burned from EPOC. I’d say if you worked really hard in an anaerobic zone where you were gasping for air, for a long period of time, you can probably add a maximum of 50 – 200 calories onto your daily caloric expenditure in addition to the calories burned during exercise.

Excess Post-Exercise Oxygen Consumption Aids Recovery

Other than getting the oxygen to your muscles that they required during anaerobic exercise when they went into an oxygen deficit, there are multiple reasons why excess post-exercise oxygen consumption occurs. This is to aid recovery.

There are two components of excess post-exercise oxygen consumption and recovery:

  • Fast component (occurs over about 20 minutes max)
  • Slow component (occurs about 24 hours max)

EPOC – Fast Component

Oxygen being breathed in during EPOC is used in recovery to:

  1. Restore muscle ATP stores – these are the molecules that are used to release energy in mitochondria
  2. Restore PC (phosphocreatine) stores – PC is used to help regenerate ATP so helps to provide more ATP to release energy
  3. Re-saturate myoglobin – these are proteins in the muscles which carry oxygen

It takes about 30 seconds to restore 50% of ATP and PC stores, whilst near complete restoration takes between 3 and 5 minutes. Re-saturation of myoglobin with oxygen takes about 2 minutes and uses 0.5 litres of oxygen.

EPOC – Slow Component

The slow component uses oxygen taken in during recovery to:

  1. Remove lactic acid
  2. Maintain breathing rate and heart rate
  3. Replenish glycogen
  4. Keep body temperature elevated

Removal of Lactic Acid

Most of the lactic acid produced during anaerobic exercise is removed with an hour of finishing exercise. Essentially oxygen is used to convert lactic acid into a compound known as pyruvate which is transported to the liver. At the liver is can then be used to produce glucose and glycogen.

50% of lactic acid is removed in the first 30 minutes of recovery, which takes around 5 – 6 litres of oxygen.

Maintaining Breathing Rate and Heart Rate

In order to get the required oxygen to the muscles efficiently to pay off the oxygen debt, breathing rate and heart rate remain elevated. This helps to get more oxygen into the lungs and pump the oxygen to the muscles quicker.

This process also aids the replenishment of ATP and PC stores, as well as the re-saturation of myoglobin and removal of lactic acid.

Glycogen Replenishment

During intense exercise, glycogen (which is converted into glucose) tends to be the first source of energy used after ATP and PC stores. Carbohydrates are the most effective at replenishing glycogen stores and consuming carbohydrates with a high glycemic index can help to replenish glycogen for your next intense, anaerobic workout. In fact, post exercise is probably the only time when I’d really suggest consuming high glycemic index foods, especially if you have another hard workout later that day or the next day.

Elevated Body Temperature

At higher temperatures, oxygen dissociates (removes itself or detaches) more readily. This allows oxygen to quickly move from the lungs to the blood to the myoglobin to the muscle tissues and cells, aiding the recovery process. In addition, by keeping body temperature elevated, your blood vessels remain vasodilated (widened), so more blood carrying oxygen can get to the muscles.

Did you know about EPOC and the afterburn effect? If you found this article useful please share and let me know what you think down below!